Fix ToString() conversion from UTF8 and remove NString.NSPinnedString

Fix ToString() conversion from UTF8. …
ToString() previously relied on PtrToStringAuto which may only support UTF-16 and ANSI. Now an explicit conversion is performed to UTF8.

Remove NString.NSPinnedString …
NSPinnedString was performing an extra retain on an already retained string and otherwise offered no additional functionality. Its use for constants would leak memory. Instead, these constants have been updated to return a String which can be implicitly converted to an autoreleased NSString as needed.